1. Field of the Invention
The present invention relates to a power steering system using hydraulic pressure.
2. Description of the Prior Art
One prior power steering device using hydraulic pressure is disclosed in U.S. Pat. No. 3,602,101. As shown in FIG. 5 of the accompanying drawings, the disclosed power steering device has a valve spool S disposed between a pair of chambers C1, C2 supplied with a working fluid from a hydraulic-pressure pump P. When an input shaft is turned in one direction, the valve spool S is slid to the right by a valve rod L to open variable orifices VO1, VO3 and close variable orifices VO2, VO4. Therefore, the working fluid flows from the chamber C1 through a passage D1 into one fluid-pressure chamber E1 of a reciprocable fluid-pressure cylinder assembly E, while the working fluid flows from the other fluid-pressure chamber E2 through passages D2, D3 back into a reservoir R. The piston E3 of the fluid-pressure cylinder assembly E is then slid to the left due to the difference between the fluid pressures in the fluid-pressure chambers E1, E2. At this time, the fluid pressures in the fluid-pressure chambers E1, E2 are transmitted respectively through first and second orifices O1, O2 in the valve spool S into reactive pressure chambers G1, G2, respectively, which are defined in the valve spool S by reactive pistons F1, F2 slidably fitted in axial bores S1, S2, respectively, in the valve spool S and fixed to a valve cylinder A. The valve spool S is therefore subject to a leftward reactive force due to the difference between the fluid pressures in the reactive pressure chambers G1, G2. The reactive pressure chambers G1, G2 communicate with the working fluid passages only through the first and second orifices O1, O2, so that when the valve spool S tends to vibrate, the working fluid flows into and out of the reactive pressure chambers G1, G2 through the orifices O1, O2 to prevent the valve spool S from vibrating.
With the conventional power steering device, therefore, the reactive force on the valve spool is produced by applying the difference between the fluid pressures in the fluid-pressure chambers in the reciprocable fluid-pressure cylinder assembly to the valve spool. Since the fluid pressure difference is large, the surface area of the valve spool on which the fluid pressure acts should be considerably smaller than the cross-sectional area of the valve spool in order to generate a reactive force of an appropriate magnitude. One solution, which could be available in a small-size power steering device, would be to form reactive pressure chambers within the valve spool, but this construction would be complex and would not be manufactured at a reduced cost. Since such reactive pressure chambers would be small in cross-sectional area, the ability to prevent the valve spool from vibrating would not be so great, and the valve spool would tend to vibrate, giving the driver an impaired feel in steering the vehicle.